This invention pertains to high performance mixed matrix membranes (MMMs) for use in gas and liquid separations. More particularly, the invention pertains to a novel method of making high performance MMMs using stabilized concentrated suspensions containing uniformly dispersed polymer stabilized molecular sieves and at least two types of polymers as the continuous blend polymer matrix.
Gas separation processes with membranes have undergone a major evolution since the introduction of the first membrane-based industrial hydrogen separation process about two decades ago. The design of new materials and efficient methods will further advance the membrane gas separation processes within the next decade.
The gas transport properties of many glassy and rubbery polymers have been measured as part of the search for materials with high permeability and high selectivity for potential use as gas separation membranes. Unfortunately, an important limitation in the development of new membranes for gas separation applications is a well-known trade-off between permeability and selectivity of polymers. By comparing the data of hundreds of different polymers, Robeson demonstrated that selectivity and permeability seem to be inseparably linked to one another, in a relation where selectivity increases as permeability decreases and vice versa.
Despite concentrated efforts to tailor polymer structure to improve separation properties; current polymeric membrane materials have seemingly reached a limit in the trade-off between productivity and selectivity. For example, many polyimide and polyetherimide glassy polymers such as Ultem® 1000 have much higher intrinsic CO2/CH4 selectivities (αCO2/CH4) (˜30 at 50° C. and 690 kPa (100 psig) pure gas tests) than that of cellulose acetate (˜22), which are more attractive for practical gas separation applications. These polymers, however, do not have outstanding permeabilities attractive for commercialization compared to current commercial cellulose acetate membrane products, in agreement with the trade-off relationship reported by Robeson. On the other hand, some inorganic membranes such as zeolite and carbon molecular sieve membranes offer much higher permeability and selectivity than polymeric membranes, but are expensive and difficult for large-scale manufacture. Therefore, it is highly desirable to provide an alternate cost-effective membrane in a position above the trade-off curves between permeability and selectivity.
Based on the need for a more efficient membrane than polymer and inorganic membranes, a new type of membrane, mixed matrix membranes (MMMs), has been developed recently. MMMs are hybrid membranes containing inorganic fillers such as molecular sieves embedded in a polymer matrix.
Mixed matrix membranes have the potential to achieve higher selectivity with equal or greater permeability compared to existing polymer membranes, while maintaining their advantages. Much of the research conducted to date on mixed matrix membranes has focused on the combination of a dispersed solid molecular sieving phase, such as molecular sieves or carbon molecular sieves, with an easily processed continuous polymer matrix. For example, see U.S. Pat. Nos. 6,626,980; 4,740,219; 5,127,925 ; 4,925,562; 4,925,459; 5,085,676; 6,663,805; 4,705,540; 4,717,393; 4,880,442; US 2004/0147796; US 2004/0107830; US 2003/0220188; US 2005/0043167; US 2002/0053284; U.S. Pat. Nos. 6,755,900; 6,500,233; 6,503,295; US 2006/0117949; US 2005/0268782; US 2005/0230305; US 2006/0107830; US 2005/0139066; and U.S. Pat. No. 6,508,860. The sieving phase in a solid/polymer mixed matrix scenario can have a selectivity that is significantly larger than the pure polymer. Addition of a small volume fraction of molecular sieves to the polymer matrix, therefore, increases the overall separation efficiency significantly. While the polymer “upper-bound” curve has been surpassed using these solid/polymer MMMs, there are still many issues that need to be addressed for large-scale industrial production of these new types of MMMs.
The first known article concerning mixed matrix membranes was published in 1960 by Barrer et al. See JOURNAL OF PHYSICAL CHEMISTRY 1960, 64, 417-21. This work reported the formation of ion exchange membranes by dispersing several different zeolites in an inert polymer resin. Voids and defects due to the poor interfacial adhesion, however, were observed at the interface of the inorganic zeolites and the organic polymer. These voids, that are much larger than the penetrating molecules, resulted in reduced overall selectivity of the mixed matrix membranes. Research has shown that the interfacial region, which is a transition phase between the continuous polymer and dispersed sieve phases, is of particular importance in forming successful MMMs.
Typical inorganic sieving phases in MMMs include various molecular sieves, carbon molecular sieves, and traditional silica. Many organic polymers, including cellulose acetate, polyvinyl acetate, polyetherimide (commercially Ultem®), polysulfone (commercial Udel®), polydimethylsiloxane, polyethersulfone, and several polyimides (including commercial Matrimid®), have been used as the continuous phase in MMMs. In recent years, significant research efforts have been focused on material compatibility and adhesion at the inorganic solid/polymer interface in order to achieve separation property enhancements over traditional polymers with MMMs. For example, Kulkarni et al. reported the use of organosilicon coupling agent functionalized molecular sieves to improve the adhesion at the sieve particle/polymer interface of the MMMs. See U.S. Pat. No. 6,508,860. Kulkarni et al. also reported the formation of MMMs with minimal macrovoids and defects by using electrostatically stabilized suspensions. See US 2006/0117949.
Despite all the research efforts, issues of material compatibility and adhesion at the inorganic solid/polymer interface in MMMs are still not completely addressed.